1. Field of the Invention
The present invention relates to a semiconductor-laser-excited solid-state laser apparatus in which a solid-state laser crystal is excited by excitation laser light emitted from a semiconductor laser unit as in an excitation light source, and emits laser light.
2. Description of the Related Art
A Currently, there are demands for increase in output power and improvement in quality of solid-state laser apparatuses. In response to these demands, a solid-state laser apparatus achieving high output power is proposed. In the proposed solid-state laser apparatus, a solid-state laser crystal of Nd:YAG, Nd:YVO4, Nd:YLF or the like is excited by excitation laser light emitted from a broad-guide semiconductor laser unit having a high power output. In addition, as a widespread technique, the laser light generated by the solid-state laser crystal may be converted into a second harmonic wave by providing a wavelength conversion element made of, for example, a nonlinear crystal or a domain-inverted LiNbO3, in an external resonator arranged outside of the solid-state laser crystal.
On the other hand, in the current semiconductor-laser-excited solid-state laser apparatuses, the excitation light source is driven under a so-called automatic power control (APC) so as to stabilize the laser oscillation. That is, a portion of output laser light is monitored and fed back to the excitation light source so as to reduce variation in the output laser light. In order to stabilize.the output laser light by the automatic power control, it is desirable that the ratio of an increase in the output of the semiconductor laser unit to an increase in the output of the solid-state laser apparatus is constant, i.e., the output of the solid-state laser apparatus monotonously increases with the increase in the output of the semiconductor laser unit.
Nevertheless, in practice, the output of the solid-state laser apparatus does not monotonously increase even when the output of the semiconductor laser unit is increased by 10% or 20%. In a typical example of the solid-state laser apparatus, the output of the solid-state laser apparatus reaches a level of saturation when the output of the semiconductor laser unit is increased by 8% over an initial driving state.
The above problem is caused by deviation of the oscillation wavelength of the semiconductor laser unit from a desired absorption peak of the solid-state laser crystal. Since a great amount of heat is generated by the semiconductor laser unit, the oscillation wavelength of the semiconductor laser unit is highly dependent on the driving current. That is, the oscillation wavelength of the semiconductor laser unit shifts toward the longer wavelength side with increase in the driving current. Consequently, the deviation of the oscillation wavelength of the semiconductor laser unit from the desired absorption peak of the solid-state laser crystal becomes great.
For example, in a known semiconductor-laser-excited solid-state laser apparatus, a solid-state laser crystal of Nd;YAG is excited by excitation laser light having a wavelength of 809 nm emitted from a semiconductor laser unit, and emits laser light having a wavelength of 946 nm. The full width at half maximum of the peak of the oscillation wavelength at which the solid-state laser crystal of Nd:YAG best absorbs light is very small, i.e., at most 10 nm. Therefore, even when the shift of the wavelength of the excitation laser light is only a few nanometers, the wavelength of the excitation laser light deviates from the desired absorption peak of the solid-state laser crystal of Nd:YAG, and therefore the excitation laser light cannot be efficiently absorbed by the solid-state laser crystal of Nd:YAG. Thus, even when the driving current (driving power) is greatly increased, the increase in the output power of the solid-state laser apparatus is often small.
In order to solve the above problem, an attempt has been made to suppress the dependence of the oscillation wavelength of the semiconductor laser unit on the driving current by enhancing radiation effect of the semiconductor laser unit during emission of high power laser light. As disclosed in Japanese Unexamined Patent Publication No. 10(1998)-190131, which is assigned to the present assignee, an attempt has been made to optimize a mechanical member which fixes a semiconductor laser unit so as to enhance radiation efficiency and reduce the dependence of the oscillation wavelength of the semiconductor laser unit on the driving current.
However, when the output power of the semiconductor laser unit is further increased, the above optimization of the mechanical member is insufficient to sufficiently reduce the dependence of the oscillation wavelength of the semiconductor laser unit on the driving current.
Japanese Patent Application No. 11(1999)-82723, which is also assigned to the present assignee, proposes a method for solving the above problems. In the proposed method, provision is made in driving of the semiconductor laser unit so that the deviation of the oscillation wavelength of the semiconductor laser unit is prevented. Nevertheless, the characteristic of the semiconductor laser unit per se has not been fundamentally improved by the method. Therefore, output loss occurs in the solid-state laser apparatus. Thus, it is not possible to further increase the output power by the method.
The object of the present invention is to provide a semiconductor-laser-excited solid-state laser apparatus in which stable automatic power control is performed, and from which high power laser light is output.
According to the present invention, there is provided a semiconductor-laser-excited solid-state laser apparatus includes a solid-state laser element and a semiconductor laser unit including a resonator. The solid-state laser element is excited by light emitted from the semiconductor laser unit, and emits laser light. The resonator length in the semiconductor laser unit is arranged to be at least 0.8 mm.
According to the present invention, the resonator length in the semiconductor laser unit is arranged to be at least 0.8 mm, which is longer than the lengths of the resonators in the semiconductor laser units in the conventional semiconductor-laser-excited solid-state laser apparatuses. Since heat is mainly generated in the resonator of the semiconductor laser unit, the area of the semiconductor laser unit which is in contact with a radiation member such as a heatsink is increased with the increase in the resonator length, and therefore ability to dissipate the heat generated in the semiconductor laser unit is enhanced. Accordingly, the dependence of the oscillation wavelength of the semiconductor laser unit on the driving current can be remarkably reduced. Thus, the wavelength of the excitation laser light does not substantially deviate from an absorption band of the solid-state laser crystal, in which the solid-state laser crystal best absorbs the excitation laser light. Thus, the solid-state laser crystal can be efficiently excited, and a stable laser output can be obtained from the semiconductor-laser-excited solid-state laser apparatus.
In addition, the substantial area of the light emitting portion of the semiconductor laser unit is increased due to the above increase in the length of the resonator. Therefore, the operating current density can be reduced. Accordingly, it is possible to prevent deterioration of the semiconductor laser unit due to damage of the light emitting portion caused by the high current density. Thus, reliability of the semiconductor-laser-excited solid-state laser apparatus can be increased.
Preferably, the resonator length in the semiconductor laser unit is at least 1 mm. It is further preferable that the length of the resonator is at least 1.5 mm.
Further, the semiconductor-laser-excited solid-state laser apparatus according to the present invention may further comprise a second resonator which is formed by the solid-state laser element and a mirror arranged outside of the solid-state laser element, and a wavelength conversion element which is arranged in the second resonator, and generates a second harmonic wave.